As one of this type of roller bearings, FIG. 9 shows a standard cylindrical roller bearing. In this bearing, flanges 91 and a roller end surface 92 have flat surface portions 93 and 94, respectively, which both extend perpendicular to the roller center axis and are configured to be brought into contact with each other. With this arrangement, when large thrust loads are applied to the bearing, since no gaps are present between the flat surface portions 93 and 94, lubricant cannot flow into between the flat surface portions, which could lead to overheating or seizure.
In order to improve the thrust load capacity of a bearing, it is necessary to prevent overheating and seizure due to excessive contact surface pressure between the roller end surfaces and the flanges while thrust loads are being applied. For this purpose, it is known to suitably determine the shapes of the surfaces of the flanges and the roller end surfaces so as to adjust the position and size of contact ovals (see JP Patent Publication 2002-195272A (especially paragraphs [0008], [0012] and [0014] of the specification)).
In particular, the roller end surface has an annular guided surface portion which can contact a flange while the roller is orbiting, wherein the annular guided surface portion is inclined obliquely, in the thrust direction toward the longitudinal center of the roller, from its edge near the roller center axis to its edge near the raceway such that the circumference of the annular guided surface portion whose center lies on the roller center axis is small. The flange has an annular guiding surface which can contact the annular guided surface portion, wherein the annular guiding surface is inclined obliquely outwardly of the bearing with respect to the thrust direction, from its edge near the raceway to its edge near the roller center axis such that the circumference of the annular guiding surface whose center lies on the bearing center axis changes. With this arrangement, since the annular guided surface portion of the roller end surface and the annular guiding surface of the flange are inclined, the annular guided surface portion and the annular guiding surface are brought into point contact or line contact with each other, so that a gap forms between these surfaces, allowing lubricant to flow smoothly into between these surfaces. This in turn prevents overheating and seizure, thus improving the thrust load capacity of the bearing.
In the bearing disclosed in JP Patent Publication 2002-195272A, the entire annular guided surface portion of the roller end surface is formed with crowning having a center of curvature located on the roller axis (whose radius of curvature is about 25 to 100 times the diameter of the roller). The crowning prevents excessive edge loads from being generated between the contact portions of the annular guided surface portion of the roller end surface and the annular guiding surface of the flange. By the combination of the radius of curvature and an acute inclination angle (about 0.15 to 0.65°) of the flange relative to the direction perpendicular to the roller center axis, it is possible to adjust the position and size of the contact oval.
In the above arrangement, although lubricant can flow smoothly, since it is necessary to operate the bearing while keeping the contact surface pressure between the annular guided surface portion of the roller end surface and the annular guiding surface of the flange within such a range that an oil film is present therebetween, there is a limit to the ability to support thrust loads. Particularly in the arrangement of JP Patent Publication 2002-195272A, in which the annular guided surface portion of the roller and the annular guiding surface of the flange are shaped such that they are brought into point contact with each other, the maximum contact surface pressure tends to be much higher than with standard bearings, so that the upper limit of the thrust load capacity of this bearing is low.